CoreData: error: Mutating a managed object after it has been removed from it's context

Hello,

I'm learning CoreData but my Tutorial is about one year old so not everything is up to date and I don't understand where the problem is. My Project includes an TableViewController with an bar button item. Here is the code

CoreDataResource.swift:

import UIKit
import CoreData

class CoreDataResource {
//    /Document
    var documentDir : URL {
        let allUrls = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
        return allUrls[0]
    }
   
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
   
    private var managedObjectModel: NSManagedObjectModel {
        guard let url = Bundle.main.url(forResource: "Listen", withExtension: "momd") else {
            fatalError("fix me: not found?")
        }
       
        guard let model = NSManagedObjectModel(contentsOf: url) else {
            fatalError("error in the model?")
        }
        return model
    }
   
    private var storeCoordinator: NSPersistentStoreCoordinator {
        let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
       
        let url = self.documentDir.appendingPathComponent("Listen.sqlite")
       
        do {
            try coordinator.addPersistentStore(ofType: NSSQLiteStoreType, configurationName: nil, at: url, options: nil)
        } catch {
            print(error)
        }
        return coordinator
    }
   
    var managedContext: NSManagedObjectContext {
        let context = NSManagedObjectContext(concurrencyType: .mainQueueConcurrencyType)
       
        context.persistentStoreCoordinator = self.storeCoordinator
       
        return context
       
    }
   
    func saveContext() {
        if managedContext.hasChanges {
            do {
                try managedContext.save()
            } catch {
                print(error)
            }
        }
    }
   
    func loadLists() -> [Liste] {
        let request: NSFetchRequest = Liste.fetchRequest()
       
        var listen = [Liste]()
       
        do{
            listen = try managedContext.fetch(request)
        } catch {
            print(error)
        }
       
        return listen
    }
   
    func newList(_ name: String) {
        let newList = NSEntityDescription.insertNewObject(forEntityName: "Liste", into: managedContext) as! Liste
       
        newList.name = name
        saveContext()
    }
}

ListTableViewController.swift:

import UIKit

class ListTableViewController: UITableViewController {
   
    let appDelegate = UIApplication.shared.delegate as! AppDelegate
   
   
    var listen = [Liste]()

    override func viewDidLoad() {
        super.viewDidLoad()
       
        reloadList()


    }
    private func reloadList() {
        listen = appDelegate.coreDataResource.loadLists()
    }
   
    @IBAction func addDummyData(_ sender: Any) {
       
        appDelegate.coreDataResource.newList("Test entry")
       
        appDelegate.coreDataResource.saveContext()
       
        reloadList()
       
    }
   
    // MARK: - Table view data source
   
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
       
        let cell = tableView.dequeueReusableCell(withIdentifier: "listCell", for: indexPath)
       
        let liste = listen[indexPath.row]
        cell.textLabel?.text = liste.name
        return cell
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        // #warning Incomplete implementation, return the number of rows
        return listen.count
    }

}

If I press in Simulator on the addDummyData button the following error occurs:

CoreData: error: Mutating a managed object 0x6000026b25c0 <x-coredata:///Liste/t8A4529E8-BD3B-4157-B3AF-E8B011B6C46E2> (0x6000005b6300) after it has been removed from its context.

I can't find the mistake. Please help me it's frustrating 😀